1. Field of the Invention
The present invention relates generally to a process of making thin film materials for solar cells and, more particularly, to a process of making thin film gallium arsenide materials for high efficiency solar cells on low-cost silicon substrates.
2. The Prior Art
For the large scale photovoltaic conversion of sunlight into electric power, very low-cost yet high efficiency solar cells are needed. Today, most solar cells are made from single-crystal silicon material. These single-crystal silicon materials are anything but low-cost, however. Furthermore, the maximum theoretical conversion efficiency for single-crystal silicon solar cells under maximum illumination with solar light on the ground at sea level (AM1) is about 23%. Solar cells made from single-crystal gallium arsenide (GaAs) materials, on the other hand, have a maximum theoretical conversion efficiency under maximum illumination on AM1 of about 27%, i.e., higher than that for single-crystal silicon cells. Because of this high efficiency and due to its physical properties, gallium arsenide comes close to the optimum for solar cell materials. Owing to the high absorption coefficient for visible light of a GaAs cell, all light is absorbed in a surface layer of the cell not more than about one millimicron (.mu.m) thick. Furthermore, at high temperatures, particularly above 100.degree. C., the performance of GaAs solar cells is better than that of silicon cells. The reason for this better performance is twofold. First, the voltage decrease with temperature is about 2.6 mV per .degree.C. of temperature increase for a GaAs cell, a voltage decrease which is less than that for a silicon cell. As a result, the power of GaAs cells decreases less per .degree.C. temperature increase than that of silicon cells. Second, the open circuit voltage of GaAs cells at room temperature is only slightly less than one volt, which is appreciably higher than that of silicon cells. Consequently, the voltage decrease as a percentage of the original voltage per .degree.C. of temperature increase is comparatively low for GaAs cells.
Material consumption per unit of GaAs solar cells is high, however, because a monocrystalline GaAs substrate is needed. In addition, high purity arsenic is a rare and expensive element while high purity gallium is not so rare although also very expensive. Gallium arsenide materials first must be transformed into the single-crystal state before high efficiency solar cells can be made therefrom. Polycrystalline GaAs films not only result in low efficiency cells but also exhibit instability. Attempts to grow thin film GaAs materials on foreign substrates so as to reduce material consumption per unit cells have been frustrated thus far primarily due to grain size limitations and substrate-film contamination. Presently known fabrication techniques, therefore, hold little promise for the economic large-scale production of GaAs materials for use as high efficiency, low-cost solar cells.